Problem description

In the indicated Plaxis versions an issue exists with the calculation of the maximum value used for the bulk modulus of water when using non-metric units. This issue is only relevant for Undrained (A or B), Consolidation and/or Transient flow calculations and only when using stiffer (sand or rock like) materials. The effects will depend on the situation.

The following is happening: with an Undrained (A or B), consolidation or transient flow analysis the program calculates a bulk modulus for the water (K_w) based on the Input value of the effective soil stiffness( K’). The program by default uses a large ratio between K_w and K’ such that the Plaxis calculation obtains realistic behaviour, but not such that the Plaxis calculation has numerical problems.

However when the Input K’ is already so high that the upscaled K_w would become larger than the real value of K_w the program limits the upscaled K_w value to the real value K_w,max. Unfortunately there is an unit conversion issue with this limiting value, causing problems when using non-metric units. Depending on the selected units the K_w,max value might either be too low or too large.

In the first case (too low K_w,max) this issue might cause too low excess pore pressure to be generated, a too slow consolidation process or result in incorrect discharges in a transient flow calculation. In the second case (too high K_w,max) the issue might cause too high excess pore pressures to be generated, a too fast consolidation process or result in incorrect discharges in a transient flow calculation.

Solution

If you are uncertain for your situation if the problem exists and what the effects might be please contact our support department.

This issue is solved since PLAXIS 2D 2012.01